Electroluminescent elements (organic electroluminescent elements) employing an organic thin film are being developed in recent years. Examples of methods for forming an organic thin film for use in an organic electroluminescent element include a vacuum deposition method and a wet film formation method.
The vacuum deposition method has an advantage that since superposition of layers by this method is easy, it is easy to improve charge injection from the anode and/or cathode or to confine excitons in the luminescent layer. The wet film formation method has advantages, for example, that no vacuum process is necessary and film formation in a larger area is easy, and that it is easy to mix and incorporate a plurality of materials having various functions into one layer (composition).
However, since superposition of layers by the wet film formation method is difficult, elements produced using the wet film formation method are inferior in driving stability to elements produced using the vacuum deposition method and have not reached a practical level at present except some of these. In particular, in layer superposition by the wet film formation method, it is possible to superpose two layers, for example, by using an organic solvent and an aqueous solvent. However, it has been difficult to superpose three or more layers by the method.
In patent document 1, a polymer having an oxetane group as a crosslinkable group as shown below has been proposed in order to overcome such problems encountered in layer superposition, and a layer obtained by coating this polymer and reacting the oxetane group is disclosed. It is disclosed therein that the layer has been rendered insoluble in organic solvents and it is therefore possible to superpose another layer on that layer. However, the element obtained by this method has had a problem that the element increases in operating voltage during voltage application thereto at a constant current or decreases in luminance stability during voltage application thereto, resulting in a short working life. Furthermore, that method has had a problem that a flat layer is not formed and the luminescent surface of the element obtained is not even.

Patent document 1 and non-patent document 1 disclose the charge-transporting polymer represented by the following structure, in which oxetane groups have been bonded to the main chain of the charge-transporting polymer through a spacer group. However, the element formed using the polymer disclosed in these documents has had a problem that the working life thereof is short.

Meanwhile, patent document 2, patent document 3, and non-patent document 2 disclose a polymer having benzocyclobutene-ring-derived groups as crosslinkable groups, as shown below. However, the polymer disclosed in these documents has had a problem that the element obtained has an increased operating voltage and has a short working life. There also has been a problem that the luminescent surface is not even and the element obtained has a reduced current efficiency.

There has hence been a desire for an organic electroluminescent element which has an even luminescent surface and has a high electric-power efficiency, low operating voltage, and long working life.